1. Field of the Invention
The present invention generally relates to a radar transponder, and especially to a radar transponder used for search and rescue.
2. Description of the Prior Art
FIG. 12 is a block diagram showing a conventional radar transponder for search and rescue. In FIG. 12, numeral 1 denotes a receiving antenna for receiving a signal from a search radar, numeral 2 denotes a field effect transistor (FET) amplifier connected to the receiving antenna 1, numeral 3 denotes a diode direct detector connected to the FET amplifier 2, numeral 4 denotes a video amplifier connected to the diode direct detector 3, numeral 5 denotes a control circuit connected to the video amplifier 4, numeral 6 denotes a transmission gate circuit connected to the control circuit 5, numeral 7 denotes a sweep signal generator connected to the control circuit 5, numeral 8 denotes a microwave oscillator connected to both the transmission gate circuit 6 and the sweep signal generator 7, numeral 9 denotes a transmitting antenna connected to the microwave oscillator 8.
The antenna 1 receives a signal transmitted by a search radar. The signal is amplified by the FET amplifier 2 and detected by the diode direct detector 3. The detected signal from the diode direct detector 3 is amplified by the video amplifier 4 to a level necessary to trigger the control circuit 5. When the control circuit 5 is triggered by the detected signal, it produces pulses indicating a transmission time and sends them to both the transmission gate circuit 6 and the sweep signal generator 7. The sweep signal generator 7 has the sawtooth wave generator 12 as, for example, shown in FIG. 13. In response to a pulse from the control circuit 5, the sweep wave generator 7 outputs the predetermined required number of sawtooth wave signals to the microwave oscillator 8. A transmission gate pulse from the transmission gate circuit 6 is inputted to the microwave oscillator 8, and the oscillator 8 oscillates for time period corresponding to the gate pulse and performs a frequency sweep in a frequency range corresponding to voltage of sawtooth wave from sweep signal generator 12. A transmission signal from the microwave oscillator 8, which sweeps in the frequency range is radiated by the transmitting antenna 9 as a radio wave.
The sawtooth wave generator 12 includes, at the output end, a parallel circuit constituted of a resistor 10 and capacitor 11 as shown in FIG. 13. Therefore, a current discharged from the capacitor 11 through a resistor 10 is generally expressed by a function exp(t) where t represents time. The sawtooth wave signal outputted from the signal generator 7 to the microwave oscillator 8 is affected by this function exp(t) and shows voltage-to-time characteristics of downward convexity in the falling portion as shown in FIG. 14. Now, assume that the microwave oscillator 8 has frequency-to-sweep voltage characteristics in a prescribed frequency range .DELTA.F as shown in the FIG. 15. When such a sawtooth wave signal as that shown in the FIG. 14 is inputted to the microwave oscillator 8, a response wave in the prescribed main sweep time period in the prescribed frequency range .DELTA.F shows frequency-to-time characteristics of downward convexity as shown in FIG. 16. Therefore, as in FIG. 16, the sweeping time .DELTA.t1 of the response wave from a radar transponder, corresponding to the frequency range .DELTA.f1, becomes shorter. Because of this, it may happen that the level of the response signal does not reach to a desired level. The search radars include many types having different property such as a receiving frequency band and some of them may not receive a response wave properly if the wave does not have a sufficient level.
Furthermore, as shown in FIG. 17, a sawtooth wave outputted from the sawtooth wave generator 12 has characteristics of S-shaped curve and accordingly a response wave in a return sweep has frequency-to-time characteristics of S-shaped curve in the prescribed main sweep time period in the prescribed frequency range .DELTA.F as shown in FIG. 18. Therefore, as shown in FIG. 18, sweeping time period .DELTA.t2 in a return sweep of a response wave, corresponding to frequency range .DELTA.f2, becomes shorter and some of the search radars may not detect the radiated response signal.
The related technology is described in the document "linearization of frequency sweep characteristics in 9 GHz radar transponder module", preliminary thesis, page G251, thesis number G7-13, presented at Kansai branch conference of electric-related society.
As explained above, in the conventional radar transponder, a sawtooth wave signal outputted from sawtooth wave generator 12 has the frequency-to-time characteristics of convexity in the falling portion or S-shaped curve in the rising portion. Accordingly, a response wave from the radar transponder has only a short frequency sweep time in the prescribed frequency range in the prescribed main sweep time period or in the prescribed return sweep and this raise a problem that some of the radars, which use a certain frequency band do not receive such response signals properly.